1. Field
The present invention is directed to novel phosphate transport inhibitors, more specifically, inhibitors of intestinal apical membrane Na/phosphate co-transport, and methods for their preparation and use as therapeutic or prophylactic agents.
2. Description of the Related Art
Patients with inadequate renal function, hypoparathyroidism, or certain other medical conditions (such as hereditary hyperphosphatemia, Albright hereditary osteodystrophy, amyloidosis, etc.) often have hyperphosphatemia, or elevated serum phosphate levels (wherein the level, for example, is more than about 6 mg/dL). Hyperphosphatemia, especially if present over extended periods of time, leads to severe abnormalities in calcium and phosphorus metabolism, often manifested by secondary hyperparathyroidism, bone disease and ectopic calcification in the cardiovascular system, joints, lungs, eyes and other soft tissues. Higher serum phosphorus levels are strongly associated with the progression of renal failure, cardiovascular calcification and mortality in end-stage renal disease (ESRD) patients. High-normal serum phosphorus levels have been associated with cardiovascular events and mortality among individuals who have chronic kidney disease (CKD) and among those who have normal kidney function (see, e.g., Joy, M. S., P. C. Karagiannis and F. W. Peyerl, Outcomes of Secondary Hyperparathyroidism in Chronic Kidney Disease and the Direct Costs of Treatment, J. Manag. Care Pharm., 13(5):397-411 (2007)) The progression of kidney disease can be slowed by reducing phosphate retention. Thus, for renal failure patients who are hyperphosphatemic and for chronic kidney disease patients who have serum phosphate levels within the normal range or only slightly elevated, therapy to reduce phosphate retention is beneficial.
For patients who experience hyperphosphatemia, calcium salts have been widely used to bind intestinal phosphate and prevent its absorption. Different types of calcium salts, including calcium carbonate, acetate, citrate, alginate, and ketoacid salts have been utilized for phosphate binding. A problem with all of these therapeutics is the hypercalcemia, which often results from absorption of high amounts of ingested calcium. Hypercalcemia causes serious side effects such as cardiac arrhythmias, renal failure, and skin and vascular calcification. Frequent monitoring of serum calcium levels is required during therapy with calcium-based phosphate binders. Other calcium and aluminum-free phosphate binders, such as sevelamer, a crosslinked polyamine polymer, have drawbacks that include the amount and frequency of dosing required to be therapeutically active. The relatively modest phosphate binding capacity of those drugs in vivo obliges patients to escalate the dose (up to 7 grs per day or more). Such quantities have been shown to produce gastrointestinal discomfort, such as dyspepsia, abdominal pain and, in some extreme cases, bowel perforation.
An alternative approach to the prevention of phosphate absorption from the intestine in patients with elevated phosphate serum levels is through inhibition of the intestinal transport system which mediates phosphate uptake in the intestine. It is understood that phosphate absorption in the upper intestine is mediated at least in part by a carrier-mediated mechanism which couples the absorption of phosphate to that of sodium. Inhibition of intestinal phosphate transport will reduce body phosphorus overload. In patients with advanced kidney disease (e.g. stage 4 and 5), the body phosphorus overload manifests itself by serum phosphate concentration above normal levels, i.e. hyperphosphatemia. Hyperphosphatemia is directly related to mortality and morbidity. Inhibition of intestinal phosphate transport will reduce serum phosphate concentration and therefore improve outcome in those patients. In chronic kidney disease patients stage 2 and 3, the body phosphorus overload does not necessarily lead to hyperphosphatemia, i.e. patients remain normophosphatemic, but there is a need to reduce body phosphorus overload even at those early stages to avoid associated bone and vascular disorders, and ultimately improve mortality rate. Similarly, inhibition of intestinal phosphate transport would be particularly advantageous in patients that have a disease that is treatable by inhibiting the uptake of phosphate from the intestines. Inhibition of phosphate absorption from the glomerular filtrate within the kidneys would also be advantageous for treating chronic renal failure. Furthermore, inhibition of phosphate transport may slow the progression of renal failure and reduce risk of cardiovascular events.
While progress has been made in this field, there remains a need in the art for novel phosphate transport inhibitors. The present invention fulfills this need and provides further related advantages.